The rare earths are a group of 17 elements consisting of the lanthanide series as well as yttrium and scandium. One of the first rare earth mines to be commercially developed is located in Mountain Pass, Calif.; originally producing a concentrate for misch-metal and europium for color televisions. After several decades dominating the market the mine began facing competition from rare earth sources located in China, most notably mines located near Baotou. Falling prices and environmental issues led to the cessation of mining operations at Mt. Pass in 1996. More recently, amid increased demand, mining operations are scheduled to be restarted, along with an advanced processing facility producing up to 40,000 tonnes of product on a rare earth oxide (REO) basis per year. As the mine, now owned by Molycorp Inc., strives to be the low cost producer an emphasis has been placed on improving past mineral and chemical processing techniques. Though the mine will primarily produce light rare earths (defined in this context as lanthanum, cerium, praseodymium and neodymium) close to 1% of REO in the mined ore will be in the form of Sm—Lu, Y. Most modern processing facilities utilize solvent extraction to separate the chemically similar rare earth elements from one another (1). This is due to the continuous nature of solvent extraction as well as advances in extractants which have led to more efficient separations. For separations on a large scale solvent extraction has proven to be an efficient process though a case can be made for separation by ion-exchange if a lower throughput is required, as is the case with the “heavies” being produced by Molycorp. Ion-exchange offers the benefits of a smaller plant footprint, the potential for lower power requirements and a more flexible flow-sheet given skid mounted columns and ease of reconfiguration.
Separation of rare earths through ion-exchange is not a novel concept; in fact it was developed prior to solvent extraction and was the first technique to be used on a large scale. The use of column chromatography using selective eluents was first reported by Frank Spedding, working at Ames Laboratory (2). However prior attempts to purify rare earths using this method suffered significant problems, for example drawbacks of this process were the complexity of its operation and the time required to process solutions. Bands of rare earths move slowly through the columns and rare earth mixtures can require months to elute through the column height required to achieve desired purity.